I would suggest a new mattress. Often times, we buy these generic mattress' for babies. Well, i learned the hard way, there are many differences between mattress'. First off, those generic mattress' have toxins hidden in the foam, and other materials, and it makes it hard for baby to breath, get tired from not being able to sleep well, or get sick. I'll give you the link to the website i found my mattress.
its called
Crib Mattress | Baby Mattress | Organic Baby Crib Mattress
its all gel. no toxins, no harmful things. i hope i helped. Good luck.